For the illumination of digital projection systems, solid state lighting could be a preferred method. Typically a light source is combined with an imaging system which yields the actual image. The brightness of the projection system is mainly depending on the power and the nature of the light source. Up to now lamps have been used more or less exclusively. For low brightness systems, UHP lamps are used, for high brightness systems, Xenon lamps are used. Lamps are expensive and often do not last long enough. However conversion to solid state lighting still remains a challenge.
LED illumination technology has been used successfully for low brightness systems. Typically LED illumination replaces the UHP lamps for low brightness systems up to 1.500 lumens. Laser illumination technology is currently engaged in development for replacement of Xenon lamps in the high brightness systems above 10 k lumens.
In between the brightnesses where LED illumination stops and Laser illumination starts, there is an opportunity for a lightsource which is scaleable within that range.
Diode lasers have been used in several applications and red and blue diode-lasers are known. Blue lasers have been developed for application in Blue Ray disk systems and are now marketed in compact packages, high power output (e.g. Nichia TO9 package) and low cost. Blue light has a wavelength which is suitable for pumping electroluminescent phosphors, e.g. the phosphors often used in LED's for general lighting applications. These phosphors are commonly available, with the one phosphor being more efficient than the other. Combining the blue lasers with light emitting phosphors is called laser-phosphor conversion or phosphor pumping.
This process is already used in several projection applications, but only at lower brightness levels.
A limitation still lies in the heat dissipation of the phosphors. A phosphor layer emits light rather in a lambertian way, which makes the light difficult to be captured and collimated. Hence the blue pump energy is projected on the phosphor in the most compact way, usually a very small spot, which increases the efficiency of capturing the light in the downstream optical system, but also increases the energy density in the phosphor layer. In this small spot, high energy dissipation will saturate the phosphor at a certain level, with brightness clipping and phosphor layer damage as a result.
In order to avoid this, the spot can be made larger, or the cooling can be made more efficient. Typically, in these applications, the phosphor material is mounted on a disk, which is composed of glass, to allow pumping from the back, or which is composed out of a metal which requires pumping from the front, but provides a better basis for transporting the dissipated heath through the color wheel's metal base. Due to the rotation of the wheel in free air, the wheel surface itself and the phosphor layer are cooled by air. Wavelength conversion methods that use excitation light produced by solid-state light source such as laser diodes (LDs) or light emitting diodes (LEDs) and wavelength conversion materials such as phosphors or quantum dots can produce high brightness light at wavelengths different from the wavelength of the excitation light. For high power white LED devices, often conversion from blue to white light is accomplished by phosphor wavelength conversion. To produce high brightness illumination in a projection device, red, green and blue solid state light sources, such as high power LEDs and lasers, offer improved lifetime over arc lamps. Lasers offer higher brightness then LEDs. When using lasers to produce high brightness illumination, a red laser is available in the art, for instance diode bars. A diode laser can also be used to produce the blue light, but disadvantageously a high power green diode laser is still way out in the future. Hence the possibility to convert blue to green light using a blue diode laser and green phosphors is quite attractive, even if this results in some conversion losses.
US20090284148 describes a projector using this principle in its illumination path. A blue light is generated by using a blue high power diode laser (for instance a Nichia™ diode laser) which is converted through a green light producing phosphor which is deposited on a rotating color wheel. The color wheel also takes care of the color sequencing needed for the single light valve illumination in the system. However, the output power of the illumination system is limited due to the power density of the excitation light projected on the phosphors. The longer the phosphor is exposed to the high density laser light, the faster the phosphor will burn-in. Conversion losses result in local heating of the phosphor, which in turn reduces its efficiency and can cause a shift in its emission spectrum. Rotation of the color wheel will distribute the power density over the color wheel area corresponding with the spot of the excitation laser falling on the color wheel over the track covered by the spot over the rotating phosphor surface.
WO2009017992 discloses a similar illumination device wherein the high power laser spot is illuminating while a color wheel is rotationally moving and as a result the phosphors comprised in that illuminated circular path are repeatedly heated.